Gordon Brown is scrambling to get his General Election campaign back on track after he yesterday described a pensioner he had just spoken to as a "bigoted woman".

Gillian Duffy (65) found herself the unwitting principal in an encounter that has already become one of the defining moments of the 2010 campaign.

The incident wrecked Gordon Brown's preparations for today's televised leaders' debate and threatens to haunt him to polling day — and beyond.

Pictures of the humbled Prime Minister arriving at Mrs Duffy's pebble-dashed terraced house in Rochdale, Lancashire, to apologise for calling the lifelong Labour supporter a “bigot” were beamed around the world.

What he said indoors during the 39-minute meeting remains unknown.

The blunder horrified Labour strategists, who had been pinning their hopes on today's debate about economic issues.

Particularly damaging appeared to be the juxtaposition between his public statement to the 65-year-old widow — “Very nice to see you” — with his private thoughts seconds later : “That was a disaster ... should never have put me with that woman ... ridiculous ... bigoted woman”.

The extraordinary chain of events began when Mrs Duffy spotted a gaggle of camera crews, journalists and Labour apparatchiks surrounding the Prime Minister, who was in the town to watch young offenders clean a cycle path.

Mrs Duffy seized the opportunity to challenge Mr Brown over the national debt, and the Labour candidate Simon Danczuk ushered her over to meet the Prime Minister.

Mrs Duffy began by telling him she was “absolutely ashamed of saying I'm Labour”, and went on to complain about the scale of the deficit, taxes paid by pensioners and university tuition fees. In passing, she also mentioned “all these eastern Europeans” heading for Britain.

The meeting ended amicably and Mrs Duffy told reporters she was definitely going to vote Labour after all.

However, seconds later Mr Brown's private thoughts — caught on the microphone attached to his lapel — were broadcast on national television, and they were very different from his public platitudes.

After he got into his limousine, the Prime Minister snapped at his adviser, Justin Forsyth, that the meeting had been a “disaster” and demanded to know whose idea it had been. As the flustered aide said he did not know, Mr Brown pinned the blame on his long-serving assistant Sue Nye.

Mrs Duffy was still only a few yards away when astonished reporters caught up with her to relay what her Prime Minister really thought of her.

The clearly upset pensioner replied: “He's going to lead this county and he has called an ordinary woman who has just come up and asked him questions — and he's calling me a bigot.”

Back at Labour headquarters in London it was clear the party had a public relations disaster on its hands.

Their horror deepened as Mr Brown was forced to listen in a Manchester studio to a recording of his remarks during an interview with Radio 2's Jeremy Vine.

As the tape played, the Prime Minister, who had obviously forgotten he was being filmed, held his head in his hands.

Watching in London, Lord Mandelson, Labour's campaign supremo, ordered his Press team to get a message to Mr Brown to warn him he was on film.

Too late: the picture of the despairing Labour leader will become an enduring image of the campaign.

The party launched a desperate damage-limitation exercise as soon as he left the studio.

The Prime Minister called her at home to apologise, telling her she was “a good woman”, while Lord Mandelson went on air to tell the world that Mr Brown was “mortified”.